Mixed assembly planning is the process of defining how a PCBA moves through SMT placement, through-hole insertion, selective soldering, hand operations, inspection, and electrical verification before the build enters controlled release.
The key engineering decision is not simply whether a board contains SMT and THT parts. The decision is which component families belong to each manufacturing branch, when each branch occurs, and what evidence confirms that the route is ready.
Quick Answer
Freeze mixed assembly by assigning every component family to a defined route: SMT reflow, post-reflow THT insertion, selective solder, wave-capable branch, or controlled manual operation. Then connect that route to the BOM, assembly drawing, inspection plan, and electrical-test requirements before RFQ or first build.
For the broader stack connecting assembly input control, inspection, test planning, and release gates, review the PCBA Assembly Test and Quality Guide.
What decisions define a mixed assembly route before production?
Engineers should review four boundaries first:
| Decision area | What must be decided | Release impact |
|---|---|---|
| Population split | Which parts are SMT, THT, press-fit, or manual | Prevents factory interpretation during setup |
| Route sequence | When insertion, soldering, inspection, and test occur | Defines process ownership |
| Special-process branch | Which joints require selective solder or manual handling | Controls thermal and access risk |
| Verification timing | When AOI, AXI, ICT, FCT, or visual review occurs | Keeps evidence aligned with the actual build |
A released BOM alone does not freeze the assembly route. The manufacturing team also needs the assembly intent behind that BOM.
What is the difference between SMT, THT, selective solder, and manual branches?
Mixed boards normally combine several manufacturing paths.
SMT route
The SMT branch usually contains components placed by automated equipment and soldered through reflow.
Typical examples:
- IC packages
- resistors and capacitors
- fine-pitch devices
- low-profile surface-mount components
The SMT route should be stabilized before later insertion steps because subsequent operations can affect access, inspection, and rework.
THT route
Through-hole components often remain necessary for:
- high-mechanical-stress connectors
- terminals
- transformers
- large capacitors
- power interfaces
The question is not whether THT is acceptable. The question is whether the selected solder and insertion method matches the component location.
Selective solder route
Selective soldering is commonly used when a board already contains reflowed SMT components but still requires controlled soldering of specific through-hole joints.
The route depends on:
- nozzle access
- component clearance
- thermal limits
- solderability
- board support
- acceptance criteria
A selective solder process is not automatically suitable for every THT location.
Manual branch
Some joints remain manual because of:
- mechanical hardware
- unusual geometry
- low-volume exceptions
- restricted automated access
Those exceptions should be visible in the assembly package rather than discovered during production.
Which mixed assembly route should be selected?
Use this decision matrix before RFQ or first build:
| Board condition | Preferred route review | Main questions |
|---|---|---|
| Mostly SMT with limited connectors | SMT + selective solder review | Can the nozzle reach every required joint? |
| Dense SMT near large THT parts | Selective solder feasibility review | Are thermal and clearance limits acceptable? |
| Large THT population with open solder side | Wave solder evaluation | Does the board and pallet concept support the exposure? |
| Unique mechanical or odd-form parts | Manual operation review | Are exceptions documented and repeatable? |
| Mixed critical power and signal regions | Route plus test review | Are inspection and electrical gates aligned? |
What failure modes appear when the route is not frozen?
| Failure mode | Root cause | Production consequence |
|---|---|---|
| Connector added late to THT branch | BOM and route ownership are unclear | Rework and delayed NPI |
| Selective solder path blocked | Nozzle access was not reviewed early | Manual solder fallback |
| SMT components too close to solder area | Thermal interaction was missed | Component stress or process limitation |
| Inspection happens at the wrong stage | Verification was separated from route planning | Incomplete evidence |
| Drawing and BOM disagree | Release package lacks alignment | Factory interpretation risk |
A compact mixed board can create difficult trade-offs when small SMT packages sit close to heavy through-hole connectors. A selective solder fountain operating above 260 C may expose nearby components to thermal stress, while excessive shielding can reduce heat transfer into the through-hole joint. The engineering objective is to define a route that balances component protection, solder formation, inspection access, and acceptance requirements.
How should BOM, drawing, and test planning be aligned?
Mixed assembly planning becomes reliable when every release document describes the same manufacturing intent.
| Release document | Primary responsibility | Mixed assembly connection |
|---|---|---|
| BOM | Component identity and population | Defines which parts belong to each branch |
| Assembly drawing | Visual route instructions and exceptions | Shows insertion order and special handling |
| Inspection plan | Evidence collection | Defines when defects can be detected |
| Electrical-test plan | Functional confirmation | Confirms the correct verification stage |
Useful companion pages:
- Assembly BOM Best Practices
- Assembly Drawing Essentials
- Design for Assembly Checklist
- Testing & Quality
- First Article Inspection
Related service paths:
What should be frozen before RFQ, first build, or production release?
A supplier-ready mixed assembly package should freeze:
- SMT, THT, selective solder, and manual population ownership
- route order for each component family
- assembly drawing notes and special instructions
- inspection and electrical-test checkpoints
- revision alignment between BOM, CAD outputs, and manufacturing documents
A board can be technically manufacturable while still lacking a controlled release route. The purpose of planning is to remove manufacturing assumptions before they become production decisions.
Next steps with APTPCB
If your board combines dense SMT placement with connectors, terminals, shields, power devices, or other through-hole content, the route should be reviewed before pilot production.
Send the Gerber or ODB++ package, BOM, assembly drawing, and preliminary route requirements to [email protected], or begin through the quote page.
APTPCB can review mixed-process route decisions including selective solder access, soldering clearance, inspection timing, and production release alignment.
FAQ
Is mixed assembly planning only about combining SMT and THT parts?
No. It also defines timing, branch ownership, inspection points, and release evidence.
Does every mixed board need selective soldering?
No. The correct route depends on component placement, access, thermal conditions, volume, and acceptance requirements.
Why does the assembly drawing matter in mixed builds?
Because insertion order, special handling, and exceptions often cannot be communicated through the BOM alone.
Can test planning be separated from assembly route planning?
No. The route determines when inspection and electrical verification provide meaningful evidence.
When is mixed assembly planning complete?
It is complete when the factory can build the board without interpreting missing route decisions.
Public references
Author and review information
- Author: APTPCB mixed-assembly planning content team
- Technical review: SMT, THT, and NPI route-planning team
- Last updated: 2026-05-13